Abstract: Mudslide monitoring is an important element of mudslide early warning. Setting a reasonable rainfall threshold is even more crucial to the effectiveness of early warning. To monitor the debris flow disaster along the Nanjian-Jingdong highway in Yunnan, three typical debris flow ditches Tianbaotian ditch, Anlihe ditch, and Zuojia ditch were selected in Nanjian, Yunnan. and the critical rainfall thresholds at different frequencies were calculated under the two conditions of general rainfall and drought in the previous period by combining with the calculation method of hydrological production and sinking and the mechanism of debris flow initiation. The results are as follows: when the previous rainfall is normal, the critical thresholds for the three debris flow gullies are 90.63 mm/h, 67.04 mm/h, and 97.23 mm/h for the red level Ⅰ; 85.22 mm/h, 59.78 mm/h, and 85.41 mm/h for the orange level Ⅱ; and 69.61 mm/h, 49.65 mm/h, and 69.46 mm/h for the yellow level Ⅲ; and 58.04 mm/h, 41.47 mm/h, and 57.39 mm/h for the blue level Ⅳ respectively. In the case of drought, the red level I critical thresholds were 99.06 mm/h, 70.65 mm/h, and 99.45 mm/h; the orange level II critical thresholds were 87.7 mm/h, 63.26 mm/h, and 87.4 mm/h; and the yellow level Ⅲ critical thresholds were 72.14 mm/h, 52.95 mm/h, and 71.10 mm/h; and the blue level Ⅳ critical thresholds were 60.18 mm/h, 44.81 mm/h, and 59.02 mm/h respectively. At different levels, the I-D curves exhibit a functional relationship of I=\alpha \times D^\beta , where the rainfall intensity decreases as the duration of the rainfall increases. The types of rainfall that can make the mudslide outbreaks are analyzed into short and rapid rainstorms, light rainfall with small amount but long duration, and sudden rainfall under the circumstance of a large amount of rainfall in the previous period.